Sources yesterday indicated to the Herald Sun that the investigation centred on drug-related activity.

It is understood the probe began in Geelong before any links were made to Thompson's property.

Mark "Bomber" Thompson. Picture: Wayne Ludbey

Friends of Thompson have indicated that he has been struggling with mental health problems.

Victoria Police yesterday told the Herald Sun: "Detectives from Southern Metro Divisional Response Unit and Geelong Divisional Response Unit executed a warrant at Port Melbourne on 5/1/18. ­Detectives attended the Rouse Street address at 9.30am ... No arrests have been made and the investigation is ongoing."

Police sources say Thompson may have become embroiled in an inquiry of which he is not the target. Sources last night could not confirm whether he had become entangled via phone intercepts.

Thompson, 54, was fined $30,000 for his part in the 2012 Essendon supplements saga.

He has described himself as being "bitter and twisted'' over the protracted scandal, blaming it for the breakdown of his marriage to second wife Jana.

Thompson, an astute businessman, last year made a reported $7 million after selling his stake in a farm at Armstrong Creek, near Mount Duneed, on the outskirts of Geelong.

The three-time premiership player's career spanned 1983 to 1996, and he led Geelong to two premierships as coach in 2007 and 2009.

He stood in for James Hird to coach Essendon in 2014.

Thompson drove a black four-wheel drive past the Rouse St property just after 5.30pm yesterday before taking off down an alleyway.

Workers next door, who asked not to be named, said police had spent a large part of Friday at the address.

"We could see them coming in and out the whole day. They left about 5pm," one man said.

"We didn't see them taking anything, but they had gloves on and they were taking photos of things. There were quite a few police cars, including undercover cops.

"I don't think he was home. I haven't seen him for a while. He just comes in occasionally."

Another neighbour, Steve, who did not want his surname published, said Thompson was not home when police arrived.

"The first eight cops had a battering ram and they just smashed his door down and went charging in. Then a second lot of eight cops went in and they smashed down another door inside. All they took out was a few evidence bags. They didn't look big.

"From time to time, you see him. You only see his vehicle here every once and a while.

"He was here on Saturday, stayed the night, and cleaned up the area."

Another said a bald man with full arm and shoulder tattoos had been living at the property and he had seen numerous visitors coming and going during the final two weeks of last year.